1. Field of the Invention
The present invention relates to a thermoplastic preform, apparatus and process for blow molding the preform into biaxially oriented, shaped articles and, more particularly, to annealing returnable polyethylene terephthalate bottles which are subjected to washing and reuse.
2. The Prior Art
Refillable plastic bottles reduce landfill and recycling problems of disposable plastic beverage bottles and, more particularly, those bottles formed from polyethylene terephthalate or PET.
A refillable plastic bottle must remain aesthetically pleasing and functional over numerous washings and refillings as discussed by U.S. Pat. Nos. 4,755,404, 4,725,464 and 5,066,528. Cracks, color changes, volume or structural change must be minimized.
U.S. Pat. No. 4,385,089, teaches how hollow biaxially oriented shaped articles are formed from intermediate products which may be sheets or other shapes when thermoformed or parisons or preforms when injection molded, injection blown or extrusion blown. The preform may be prepared and immediately used hot or may be stored and reheated later to a temperature having sufficient elasticity to be shaped into a bottle or other form by drawing and blowing in a cooled mold to obtain the final shape of the article. The preform is next often subjected to a heat treatment at well above the glass transition temperature of the thermoplastic to increase the articles strength and resistance to gas loss. Heat treatment also prevents distortion when the bottle is reused, including distortion during a hot caustic wash.
For other heat treating patents, see U.S. Pat. No. 4,233,022 and patents cited therein. The '022 patent teaches use of a blow mold comprising four sectional members each separated by insulating portions used to heat treat blown bottles at a temperature within the range of 150.degree. C. to 220.degree. C.
U.S. Pat. No. 5,085,822, teaches it is old to blow in a mold at 130.degree. C. and cool to 100.degree. C. to prevent deformation on removal of the container from the mold. Also taught is to retain a container in the blow mold and heat to remove stress and thereafter transferring the deformable container to a separate cooled mold to solidity. The "822" patent holds the molded container for a predetermined period of time to heat set the container followed by introducing a cooling fluid into the bottle. Also disclosed is heat setting a blown container in a separate mold.
U.S. Pat. No. 4,505,664, teaches transporting the blowing cavity and blown article to a second station where medium is circulated through the article.
U.S. Pat. No. 4,488,279, biaxially orients the article which can then be heat set.
U.S. Pat. No. 5,080,855, teaches blow molded articles which may be heat set in a second mold. Also, see also U.S. Pat. No. 4,485,134, 4,871,507 and 4,463,121 which discuss heat treating biaxially oriented bottles.
U.S. Pat. No. 4,572,811, teaches heat treating a PET container to form a spherulite, opaque texture which we have found leads to stress cracking when bottles are recycled.
U.S. Pat. No. 4,588,620, teaches preforms having a thinner bottom wall and which permit longer or deeper stretch of the shoulder and sidewall portions.
While it is known to biaxial stretch a preform using pressure, we have found the annealing of-the blown preform to be a critical factor in producing carbonated beverage bottles that are refillable.